The present invention relates to a method of producing an anisotropic cylindrical magnet by compacting a ferromagnetic powder in a magnetic field.
Nowadays, dynamic electric machines such as generators, motors and so forth incorporating permanent magnets find various uses such as a motor for driving the magnetic disk of a computer and a motor for controlling the printer attached to the computer. For such uses, a motor called "PM type of stepping motor", having a rotor constituted by a multipole cylindrical permanent magnet, is most suitably used. In fact, there is an increasing demand for this type of motor, because of its excellent controllability. Usually, the cylindrical permanent magnet used in this motor has four or more poles, and rotors having magnetic poles greater than 8, e.g. 12, 24 or 36 poles, are becoming popular.
Hitherto, isotropic ferrite magnets have been used most popularly as the cylindrical permanent magnet of the kind described. This magnet, however, cannot provide satisfactory magnetic properties. For instance, a cylindrical permanent magnet of this type, having 24 poles and being 26 mm in outside diameter, exhibits a surface magnetic flux density Bo which is as small as 900 to 950 G. A radially anisotropic ferrite magnet, produced by a process making use of rolling induced anisotropy, is proposed in, for example, U.S. Pat. No. 4,057,606. This magnet also shows unsatisfactory magnetic properties due to the use of a binder agent for rolling and winding. For instance, a cylindrical permanent magnet of this type, having 24 poles and being 26 mm in outside dia., shows only a small surface magnetic flux density Bo of 950 to 1050 G.
Under these circumstances, the present invention aims as its primary object at providing a cylindrical permanent magnet having excellent magnetic properties to obviate the problems of the prior art.
As a cylindrical permanent magnet for the PM type of stepping motor, a cylindrical permanent magnet having multipole anisotropy is only required on its surface (see Japanese Patent Application Laid-Open Publication No. 199205/82).
On the other hand, various methods have been proposed for producing cylindrical permanent magnet having radial anisotropy. Examples of such methods are shown, for example, in Japanese Patent Application Laid-Open Publication No. 74907/81 or Japanese Patent Application Laid-Open Publication No. 98402/81. However, almost no study has been made up to now as to production methods for producing a permanent magnet having multipole surface anisotropy, and the present applicant is the only firm which is known to produce this type of magnet on a mass production basis.
The term "surface anisotropy" is used in this specification to mean such a state that the axes of easy magnetization are arrayed along the line (usually an arc) which connects the poles of opposite polarities existing on a same surface, e.g. the outer peripheral surface, of the cylindrical compact or magnet.
It has been thought that a permanent magnet having surface anisotropy may be produced by compacting conducted under the influence of a magnetic field. This method, however, cannot provide sufficiently high magnetic properties and tends to cause non-uniformity of the magnetic flux density along the length of each magnetic pole, unless a special compacting method is employed. In this type of permanent magnet, slight fluctuation in magnetic flux density (of the order of 2% or less) along the length of the magnetic pole does not matter substantially and, hence, is acceptable.